"We completed a successful survey of massive scale, and what we learned is deeply disturbing," said Paul Allen, the Microsoft co-founder and billionaire philanthropist who spent $7 million (6.3 million euros) funding the census.
The first-of-its-kind survey is the largest wildlife census ever and involved flying over 18 African countries with scientists and conservationists counting live elephants and carcases to establish a baseline for future studies of elephant populations and how to protect them better.
"Armed with this knowledge of dramatically declining elephant populations, we share a collective responsibility to take action," Allen said as the results were published on Thursday at a meeting of the International Union for Conservation of Nature (IUCN) in Hawaii.
Named the Great Elephant Census (GEC), the three-year programme began in December 2013 and involved 81 aeroplanes and 286 crew flying 463,000 kilometres over 18 countries, said James Deutsch, of Allen's Vulcan Inc investment company.
A total of 352,271 elephants were counted during the survey, representing a decline of 30 percent between 2007-14 equivalent to 144,000 elephants. Currently savannah elephant numbers are declining at eight percent a year, the study said.
However, populations were found to be stable or even increasing in South Africa, Botswana, Uganda, parts of Kenya, Zambia, Zimbabwe, Malawi and the W-Arli-Pendjari conservation area that spans the borders of Benin, Niger and Burkina Faso.
"If we can't save the African elephant, what is the hope for conserving the rest of Africa's wildlife?" said Mike Chase, of conservation organisation Elephants Without Borders, who led the census.
Two countries are still to be surveyed: Central African Republic and South Sudan where conflict has made access tricky. Allen now also plans to launch a similar survey of Africa's forest elephants which are thought to have also suffered badly from poaching.